Besides conventional drying and filtering wires, the word `wire` is here intended to cover also base fabrics for press felts, different kinds of conveyor belts and other such products of a wire-like structure.
In such a spiral wire, the spirals are interconnected by means of connecting or link threads which extend through the loops of adjacent spirals. In order to ensure the cohesion of the wire thus obtained, the link threads and the loops of the sprials must be prevented from moving with respect to each other in the axial direction of the link thread.
It is previously well known to subject a drying wire woven of a synthetic thread material to a so called thermal fixation, in which the thread material is heated to a temperature near the softening temperature of the thermoplastic thread and the wire is simultaneously stretched into a desired length. In this way, the thread material can be stabilized so that the thread maintains the shape thereof and the wire maintains its length as well as possible during operating conditions.
It is mentioned in Finnish Patent Application No. 801,672, which corresponds to German Patent Application No. 2,921,491, filed on May 26, 1979, that when the link thread is of a synthetic thermoplastic material, the link thread is deformed during the thermal fixation into a wave-like shape in such a manner that bends are formed in the link thread which keep the loops of the spirals in place with respect to the link thread.
Such a fixing of the link threads and the loops is based on a deformation of the entire link thread into a plurality of bends. Consequently, the formed bends are relatively smooth, wherefore the edges of the wire easily fray, when the edge loops slip over the smooth bends of the link threads. It has also been observed that the deformations of the link threads caused by the thermal fixation when the threads are drawn into bends diminish the original resistance to hydrolysis of the threads. In addition, the edges of the wire may be damaged as a result of a weakening of the link thread at the edge of the wire, where the hydrolysis is usually more rapid due to the higher temperature at the edge of the wire as compared with the center of the wire. Such damage is one of the most important disadvantages of a spiral wire and a need therefore exists for a method that provides a wire free of such problems.